Ever since Joule for the first time demonstrated the magnetostriction in 1842 and Villari the inverse phenomenon in 1865, the so called Villari effect, many measuring devices and methods have been suggested which have utilized the coupling between the magnetic flux and the change in the mechanical state. Common to all known such measuring devices and methods is that the average length of the magnetic circuit is allowed to vary with the change in the mechanical state. As is known, the magnetizing force is inversely proportional to this average length. Since the magnetization apart from the change in the mechanical state as well as variations in the average length of the magnetic circuit in these measuring devices.
The object of the present invention is to eliminate the above mentioned double dependence of the magnetization so that it only depends on the change in the mechanical state. This is achieved by keeping the magnetizing force (H.sub.0) and the average length (l) of the magnetic circuit essentially constant and independent of the change in the mechanical state when measuring or detecting. The flux consists partly of a main flux (.phi..sub.0) associated with the driving magnetic force (H.sub.0) having a definite direction and being of such a magnitude that the magnetic properties of the body depart from the region of irreversibility, and partly of a superposed alternating flux (.phi..sub.v) which gradually vanishes. This alternating flux must have such an initial magnitude that saturation is achieved in the two directions of the alternating flux (.phi..sub.v) i.e. so that in a way known per se a point on the anhysteretic curve is reached when the alternating flux (.phi..sub.v) has vanished. After the vanishing of the alternating flux (.phi..sub.v), the disturbance which is generated by the change in the mechanical state, is registered as a voltage which in turn is induced by the change in the flux corresponding to disturbance.